SSHFP SHA-256 and ECDSA: Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records
Two new numbers for SSHFP DNS records: one for ECDSA keys and one for SHA-256 fingerprints.
Sury, O., "Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records", RFC 6594, DOI 10.17487/RFC6594, April 2012, <https://www.rfc-editor.org/info/rfc6594>.
@misc{rfc6594,
series = {Request for Comments},
number = 6594,
howpublished = {RFC 6594},
publisher = {RFC Editor},
doi = {10.17487/RFC6594},
url = {https://www.rfc-editor.org/info/rfc6594},
author = {O. Sury},
title = {{Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records}},
pagetotal = 9,
year = 2012,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) O. Sury
Request for Comments: 6594 CZ.NIC
Category: Standards Track April 2012
ISSN: 2070-1721
Use of the SHA-256 Algorithm with RSA,
Digital Signature Algorithm (DSA),
and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records
Abstract
This document updates the IANA registries in RFC 4255, which defines
SSHFP, a DNS Resource Record (RR) that contains a standard Secure
Shell (SSH) key fingerprint used to verify SSH host keys using DNS
Security Extensions (DNSSEC). This document defines additional
options supporting SSH public keys applying the Elliptic Curve
Digital Signature Algorithm (ECDSA) and the implementation of
fingerprints computed using the SHA-256 message digest algorithm in
SSHFP Resource Records.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
fInternet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6594.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Requirements Language ...........................................4
3. SSHFP Resource Records ..........................................4
3.1. SSHFP Fingerprint Type Specification .......................4
3.1.1. SHA-256 SSHFP Fingerprint Type Specification ........4
3.2. SSHFP Algorithm Number Specification .......................4
3.2.1. ECDSA SSHFP Algorithm Number Specification ..........4
4. Implementation Considerations ...................................4
4.1. Support for SHA-256 Fingerprints ...........................4
4.2. Support for ECDSA ..........................................4
5. Examples ........................................................5
5.1. RSA Public Key .............................................5
5.1.1. RSA Public Key with SHA1 Fingerprint ................5
5.1.2. RSA Public Key with SHA-256 Fingerprint .............5
5.2. DSA Public Key .............................................6
5.2.1. DSA Public Key with SHA1 Fingerprint ................6
5.2.2. DSA Public Key with SHA-256 Fingerprint .............6
5.3. ECDSA Public Key ...........................................6
5.3.1. ECDSA Public Key with SHA1 Fingerprint ..............7
5.3.2. ECDSA Public Key with SHA-256 Fingerprint ...........7
6. IANA Considerations .............................................7
6.1. SSHFP RR Types for Public Key Algorithms ...................7
6.2. SSHFP RR Types for Fingerprint Types .......................7
7. Security Considerations .........................................8
8. References ......................................................8
8.1. Normative References .......................................8
8.2. Informative References .....................................9
1. Introduction
The Domain Name System (DNS) is the global, hierarchical distributed
database for Internet naming. The Secure Shell (SSH) is a protocol
for secure remote login and other secure network services over an
insecure network. RFC 4253 [RFC4253] defines Public Key Algorithms
for the Secure Shell server public keys.
The DNS has been extended to store fingerprints in a DNS Resource
Record named SSHFP [RFC4255], which provides out-of-band verification
by looking up a fingerprint of the server public key in the DNS
[RFC1034][RFC1035] and using Domain Name System Security Extensions
(DNSSEC) [RFC4033][RFC4034][RFC4035] to verify the lookup.
RFC 4255 [RFC4255] describes how to store the cryptographic
fingerprint of SSH public keys in SSHFP Resource Records. SSHFP
Resource Records contain the fingerprint and two index numbers
identifying the cryptographic algorithms used:
1. to link the fingerprinted public key with the corresponding
private key, and
2. to derive the message digest stored as the fingerprint in the
record.
RFC 4255 [RFC4255] then specifies lists of cryptographic algorithms
and the corresponding index numbers used to identify them in SSHFP
Resource Records.
This document updates the IANA registry "SSHFP RR Types for public
key algorithms" and "SSHFP RR types for fingerprint types"
[SSHFPVALS] by adding a new option in each list:
o the Elliptic Curve Digital Signature Algorithm (ECDSA) [RFC6090],
which has been added to the Secure Shell Public Key list by RFC
5656 [RFC5656] in the public key algorithms list, and
o the SHA-256 algorithm [FIPS.180-3.2008] in the SSHFP fingerprint
type list.
Familiarity with DNSSEC, SSH Protocol [RFC4251][RFC4253][RFC4250],
SSHFP [RFC4255], and the SHA-2 [FIPS.180-3.2008] family of algorithms
is assumed in this document.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. SSHFP Resource Records
The format of the SSHFP RR can be found in RFC 4255 [RFC4255].
3.1. SSHFP Fingerprint Type Specification
The fingerprint type octet identifies the message digest algorithm
used to calculate the fingerprint of the public key.
3.1.1. SHA-256 SSHFP Fingerprint Type Specification
SHA-256 fingerprints of the public keys are stored in SSHFP Resource
Records with the fingerprint type 2.
3.2. SSHFP Algorithm Number Specification
The SSHFP Resource Record algorithm number octet describes the
algorithm of the public key.
3.2.1. ECDSA SSHFP Algorithm Number Specification
ECDSA public keys are stored in SSHFP Resource Records with the
algorithm number 3.
4. Implementation Considerations
4.1. Support for SHA-256 Fingerprints
SSHFP-aware Secure Shell implementations SHOULD support the SHA-256
fingerprints for verification of the public key. Secure Shell
implementations that support SHA-256 fingerprints MUST prefer a SHA-
256 fingerprint over SHA-1 if both are available for a server. If
the SHA-256 fingerprint is tested and does not match the SSH public
key received from the SSH server, then the key MUST be rejected
rather than testing the alternative SHA-1 fingerprint.
4.2. Support for ECDSA
SSHFP-aware Secure Shell implementations that also implement the
ECDSA for the public key SHOULD support SSHFP fingerprints for ECDSA
public keys.
5. Examples
The following examples provide reference for both the newly defined
value for ECDSA and the use of the SHA-256 fingerprint combined with
both the new and the existing algorithm numbers.
5.1. RSA Public Key
A public key with the following value in OpenSSH format [RFC4716]
would appear as follows:
---- BEGIN SSH2 PUBLIC KEY ----
AAAAB3NzaC1yc2EAAAADAQABAAABAQDCUR4JOhxTinzq7QO3bQXW4jmPCCulFsnh
8Yi7MKwpMnd96+T7uV7nEwy+6+GWYu98IxFJByIjFXX/a6BXDp3878wezH1DZ2tN
D/tu/eudz6ErpTFYmnVLyEDARYSzVBNQuIK1UDqvvB6KffJcyt78FpwW27euGkqE
kam7GaurPRAgwXehDB/gMwRtXVRZ+13zYWkAmAY+5OAWVmdXuQVm5kjlvcNzto2H
3m3nqJtD4J9L1lKPuSVVqwJr4/6hibXJkQEvWpUvdOAUw3frKpNwa932fXFk3ke4
rsDjQ/W8GyleMtK3Tx8tE4z1wuowXtYe6Ba8q3LAPs/m2S4pUscx
---- END SSH2 PUBLIC KEY ----
5.1.1. RSA Public Key with SHA1 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 1 1 ( dd465c09cfa51fb45020cc83316fff
21b9ec74ac )
5.1.2. RSA Public Key with SHA-256 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 1 2 ( b049f950d1397b8fee6a61e4d14a9a
cdc4721e084eff5460bbed80cfaa2c
e2cb )
5.2. DSA Public Key
A public key with the following value in OpenSSH format would appear
as follows:
---- BEGIN SSH2 PUBLIC KEY ----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---- END SSH2 PUBLIC KEY ----
5.2.1. DSA Public Key with SHA1 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 2 1 ( 3b6ba6110f5ffcd29469fc1ec2ee25
d61718badd )
5.2.2. DSA Public Key with SHA-256 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 2 2 ( f9b8a6a460639306f1b38910456a6a
e1018a253c47ecec12db77d7a0878b
4d83 )
5.3. ECDSA Public Key
A public key with the following value in OpenSSH format would appear
as follows:
---- BEGIN SSH2 PUBLIC KEY ----
AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBAD+9COUiX7W
YgcvIOdI8+djdoFDVUTxNrcog8sSYdbIzeG+bYdsssvcyy/nRfVhXC5QBCk8IThq
s7D4/lFxX5g=
---- END SSH2 PUBLIC KEY ----
5.3.1. ECDSA Public Key with SHA1 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 3 1 ( c64607a28c5300fec1180b6e417b92
2943cffcdd )
5.3.2. ECDSA Public Key with SHA-256 Fingerprint
The SSHFP Resource Record for this key would be:
server.example.net IN SSHFP 3 2 ( 821eb6c1c98d9cc827ab7f456304c0
f14785b7008d9e8646a8519de80849
afc7 )
6. IANA Considerations
This document updates the IANA registry "SSHFP RR Types for public
key algorithms" and "SSHFP RR types for fingerprint types"
[SSHFPVALS].
6.1. SSHFP RR Types for Public Key Algorithms
The following entries have been added to the "SSHFP RR Types for
public key algorithms" registry:
+-------+-------------+------------+
| Value | Description | Reference |
+-------+-------------+------------+
| 3 | ECDSA | [This doc] |
+-------+-------------+------------+
6.2. SSHFP RR Types for Fingerprint Types
The following entries have been added to the "SSHFP RR types for
fingerprint types" registry:
+-------+-------------+------------+
| Value | Description | Reference |
+-------+-------------+------------+
| 2 | SHA-256 | [This doc] |
+-------+-------------+------------+
7. Security Considerations
Please see the security considerations in [RFC4255] for SSHFP
Resource Records and [RFC5656] for the ECDSA.
Users of SSHFP are encouraged to deploy SHA-256 as soon as
implementations allow for it. The SHA-2 family of algorithms is
widely believed to be more resilient to attack than SHA-1, and
confidence in SHA-1's strength is being eroded by recently announced
attacks [IACR2007/474]. Regardless of whether or not the attacks on
SHA-1 will affect SSHFP, it is believed (at the time of this writing)
that SHA-256 is the better choice for use in SSHFP records.
SHA-256 is considered sufficiently strong for the immediate future,
but predictions about future development in cryptography and
cryptanalysis are beyond the scope of this document.
8. References
8.1. Normative References
[FIPS.180-3.2008]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-3, October 2008,
<http://csrc.nist.gov/publications/fips/fips180-3/
fips180-3_final.pdf>.
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, December 2009.
8.2. Informative References
[IACR2007/474]
Cochran, M., "Notes on the Wang et al. 2^63 SHA-1
Differential Path", IACR 2007/474,
<http://eprint.iacr.org/2007/474.pdf>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4716] Galbraith, J. and R. Thayer, "The Secure Shell (SSH)
Public Key File Format", RFC 4716, November 2006.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental
Elliptic Curve Cryptography Algorithms", RFC 6090,
February 2011.
[SSHFPVALS] IANA, "DNS SSHFP Resource Records Parameters",
<http://www.iana.org/assignments/
dns-sshfp-rr-parameters>.
Author's Address
Ondrej Sury
CZ.NIC
Americka 23
120 00 Praha 2
Czech Republic
Phone: +420 222 745 110
EMail: ondrej.sury@nic.cz
AI ELI5
On this page
§1Introduction
The SSHFP resource record, defined in RFC 4255, stores a fingerprint of an SSH server's public key in DNS. A client can then check the key the server presents against the fingerprint it finds in the DNS, using DNSSEC to verify the lookup.
Each SSHFP record holds the fingerprint plus two index numbers. One identifies the public key algorithm, which links the fingerprinted key to its private key. The other identifies the message digest algorithm that produced the fingerprint. RFC 4255 lists the algorithms and their numbers.
This RFC adds one entry to each list: the Elliptic Curve Digital Signature Algorithm (ECDSA) for keys, and SHA-256 for fingerprints. The text assumes you already know DNSSEC, SSH, SSHFP and the SHA-2 family.
A computer can publish a short code for its secret key in a public phone book. Both the way of naming the key and the way of making the short code each have a numbered list, and this adds one new item to each.
§2The Two New Values
The record format itself is unchanged and still comes from RFC 4255. This RFC only defines two new numbers.
- Fingerprint type 2 means the fingerprint of the public key was computed with SHA-256.
- Algorithm number 3 means the public key is an ECDSA key.
The fingerprint type octet says which message digest algorithm was used. The algorithm number octet says which kind of public key the record describes.
Each record has two little number tags. Tag 2 on the code says it was made the stronger way, and tag 3 on the key says it is the newer kind of key.
- 3ECDSA — Algorithm number: the record describes an ECDSA public key.
- 2SHA-256 — Fingerprint type: the fingerprint was computed with SHA-256.
§3What Implementations Should Do
SSHFP-aware SSH implementations SHOULD support SHA-256 fingerprints for verifying the public key. If an implementation does support them, it MUST prefer a SHA-256 fingerprint over SHA-1 when both are available for a server.
There is a strict rule for a mismatch: if the SHA-256 fingerprint is tested and does not match the key received from the SSH server, the key MUST be rejected. The client must not fall back to testing the SHA-1 fingerprint, since that would let an attacker pick the weaker check.
Implementations that support ECDSA for the SSH public key SHOULD also support SSHFP fingerprints for ECDSA keys.
If a server shows two codes, a strong one and a weak one, check the strong one. If the strong one does not match, do not give it a second chance with the weak one. Just say no.
§4Worked Examples
The RFC gives an RSA key, a DSA key and an ECDSA key, each in OpenSSH public key format, and shows the SSHFP record for each with a SHA-1 fingerprint and with a SHA-256 fingerprint. The records are written for the host server.example.net.
The two numbers after SSHFP are the algorithm and the fingerprint type. RSA uses 1, DSA uses 2 and ECDSA uses 3, so the ECDSA key with a SHA-256 fingerprint is SSHFP 3 2. SHA-1 fingerprints use type 1.
The examples show the same three keys written down with the old short code and the new stronger one, so you can see the number tags in action.
server.example.net IN SSHFP 3 2 ( 821eb6c1c98d9cc827ab7f456304c0
f14785b7008d9e8646a8519de80849
afc7 )§5IANA Registry Updates
Two IANA registries were updated. In the registry "SSHFP RR Types for public key algorithms", value 3 was added with the description ECDSA. In the registry "SSHFP RR types for fingerprint types", value 2 was added with the description SHA-256. Both entries reference this document.
There is an official list of what each number means. This RFC asked for the new numbers to be written into it.
§6Security Considerations
The RFC points to the security considerations in RFC 4255 for SSHFP records and in RFC 5656 for ECDSA, and adds advice about the digest. Users of SSHFP are encouraged to deploy SHA-256 as soon as implementations allow it.
The SHA-2 family is widely believed to be more resilient to attack than SHA-1, and confidence in SHA-1 was being eroded by recently announced attacks. The RFC says that regardless of whether those attacks affect SSHFP, SHA-256 is believed to be the better choice. It also says SHA-256 is considered sufficiently strong for the immediate future, and that predicting later developments in cryptography is beyond its scope.
The old way of making the short code is starting to look a little wobbly, so people are asked to move to the sturdier one as soon as their software can.